For purpose of lowering the combustion temperature of a diesel engine to suppress generation of NOx, so-called EGR (Exhaust Gas Recirculation) systems, which deliver part of exhaust gas discharged from the engine to an intake line, have typically been known.
In particular, in such EGR systems, there has typically been known a system in which a variable geometry turbocharger is disposed at a downstream side of an air cleaner through which outside air is supplied, so that the air supplied through the air cleaner is compressed and supplied to the engine. The respective opening degrees of an EGR valve and a nozzle of the variable geometry turbocharger are electronically controlled by a controller.
The control by the controller is performed based on a flow rate of the air filtered through the air cleaner measured by using a mass flowmeter. Thus, an accuracy of a flow rate value measured by using the mass flowmeter has a large influence on the accuracy of the control by the controller.